Metal containers of aerosol are largely grouped into two-piece cans and three-piece cans. The two-piece can is a can structured by two segments, namely the can body integrated with the can bottom and the can end. The three-piece can is a can structured by three segments, namely the can body, the top end, and the bottom end. The two-piece can has no seam (welded part) so that it gives beautiful appearance. However, the two-piece can generally requires high strain. Since the three-piece can has the seam, it is inferior in appearance to the two-piece can. The three-piece can, however, generally requires low strain. Therefore, the two-piece can is widely used for small capacity and high grade goods in the market, and the three-piece can is generally used for large capacity and low price goods.
The metal base material for an aerosol two-piece can usually adopts expensive and thick aluminum sheet, and rarely uses steel sheet base material such as inexpensive and thin sheet, including tinplate and tin-free steel. The reason is that, since the aerosol two-piece can requires high strain, drawing and DI working are difficult to apply, while aluminum allows applying impact-molding applicable to soft metallic materials. In this situation, if the steel sheet base material such as tinplate and tin-free steel which are inexpensive and high strength even with a thin sheet thickness is applicable, the industrial significance becomes remarkably high.
Although there were many proposals of drawing and DI working methods of laminated steel sheet, there is no proposal of the method for manufacturing cans such as an aerosol two-piece can of large drawing ratio and high elongation in the can height direction.
For example, Examined Japanese Patent Publication No. 7-106394, Japanese Patent No. 2526725 and Japanese Patent Laid-Open No. 2004-148324 disclose the working methods for drawing and drawing-ironing for resin-laminated metal sheet. The strain level described in Examined Japanese Patent Publication No. 7-106394, Japanese Patent No. 2526725 and Japanese Patent Laid-Open No. 2004-148324, (drawing ratio in Examined Japanese Patent Publication No. 7-106394, Japanese Patent No. 2526725 and Japanese Patent Laid-Open No. 2004-148324), is lower than the range specified. This is because Examined Japanese Patent Publication No. 7-106394, Japanese Patent No. 2526725 and Japanese Patent Laid-Open No. 2004-148324 place the target to beverage cans, food cans, and the like, and beverage cans and food cans are the cans requiring lower strain than the desired range of strain level.
Japanese Patent No. 2526725 and Japanese Patent Laid-Open No. 2004-148324 describe that, aiming to gain the prevention of delamination of resin layer and the barrier property after working, a heat treatment is applied during working and/or at an interim stage of working, or at the final stage. Japanese Patent No. 2526725 uses an orientating thermoplastic resin, and Japanese Patent Laid-Open No. 2004-148324 uses a compound of saturated polyester and ionomer.
Examined Japanese Patent Publications Nos. 59-35344 and 61-22626 describe methods of relaxing internal stress mainly by applying heat treatment at or above the melting point of the resin, and describe the application of heat treatment at a stage after the can-forming. The strain level of the can is low suggested by the detailed description and by the description of examples.
Japanese Patent No. 2526725 proposes heat treatment to relax the internal stress and to enhance the orientation crystallization, which method has become common to beverage can and the like. Although Japanese Patent No. 2526725 does not give detailed description, the temperature of heat treatment is presumably at or below the melting point since the orientation crystallization is accelerated at or below the melting point. The description and the examples of Japanese Patent No. 2526725 show that the strain level is lower than the strain level specified.
Conventional technologies did not provide methods for manufacturing cans such as aerosol two-piece cans using laminated steel sheet applying high strain. Thus, we fabricated two-piece cans using laminated steel sheet applying high strain of the steps of drawing-ironing of the laminated steel sheet to form into a shape of a cylinder integrated with a bottom, followed by diametral reduction in the vicinity of opening of the cylinder, and found the occurrence of problems characteristic to high strain, specifically the problem of delamination and fracture of resin layer. Our efforts revealed the effectiveness of the heat treatment in qualitative view. However, sole heat treatment was not sufficient, and the delamination of resin layer unavoidably appeared in a zone of high strain. As a result, simple application of the related art did not solve the problem of delamination of the resin layer. In addition, there appeared a problem of deterioration of formability of the resin layer during the forming after the heat treatment.
It could therefore be advantageous to provide a method for producing a two-piece can in which delamination and fracture of a laminate resin layer can be prevented even when a can body of a high strain level such as an aerosol two-piece can is produced.
It could also be advantageous to provide a can body of a high strain level such as an aerosol two-piece can using a laminated steel sheet.